The present invention relates to an image forming material that can be formed into an image by electro-photography, thermal transfer recording, writing tools or printing, and that can be decolored by control of thermal hysteresis or by contact with a solvent.
With the recent progress of office automation, the amounts of various pieces of information are significantly increasing, and information output is increasing accordingly. Information output is represented by display output and hard copy output from printers to paper sheets. However, display output requires a large-scale circuit board in a display unit and hence has problems in portability and cost. Hard copy output uses a large amount of paper as a recording medium when the information amount increases, leading to problems about the protection of resources. In addition, in order to recycle paper sheets on which images are printed with a printer or a copier, it is necessary to use a large amount of bleaching agent and water, which brings about a rise in recycling cost. Thus, it is proposed to reduce the consumption of paper sheets substantially by printing an image on a paper sheet using a decolorable image forming material, decoloring the formed image to return the paper sheet to a blank sheet, reusing the blank sheet, and recycling the sheet at the time when damage of the sheet becomes serious so that the sheet cannot be reused.
Recently, rewritable paper has been proposed in order to reuse hard copy paper sheets. However, this rewritable paper is applicable only to thermal recording and is not recyclable because it is made of special paper, although it is reusable.
Heretofore, an image forming material that can be decolored by heating has been proposed in, for example, Published Unexamined Japanese Patent Application No. 7-81236. The image forming material includes a color former such as a leuco dye, a developer, and an organophosphoric compound having decoloring ability. When such an image forming material is used, however, decoloration is insufficient and, as a result, a paper sheet is hard to return to the blank state.
We have been developing a decolorable image forming material in which a color former, a developer and a decolorant having high affinity with the developer are dispersed in a binder resin. Now, it becomes possible to attain an excellent decolored state in various material systems by selecting an appropriate decolorant. However, a complete decolored state is not always obtained in all material systems. Thus, it has been recognized that components of the decolorable image forming material can be desirably selected from a wide variety of materials.